Embodiments of the invention relate generally to diagnostic imaging and, more particularly, to a system and method for improved spatial resolution of a multi-slice imaging system.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. In particular, an x-ray tube included in the x-ray source emits the x-ray beam at a focal point or focal spots. The beam, after being attenuated by the subject, impinges upon an array of radiation or x-ray detectors.
In known CT systems, the x-ray beam is projected from the x-ray source through a pre-patient collimator that defines the x-ray beam profile in the patient axis, or z-axis. The collimator typically includes an x-ray-absorbing material with an aperture therein for restricting the x-ray beam.
X-ray detectors also typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom. Alternatively, x-ray detectors may include a direct conversion device that convert x-ray beams directly to electrical signals.
Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for calibration and image reconstruction.
The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject so that the angle at which the x-ray beam intersects the subject is constantly changing. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the subject comprises a set of views made at different gantry angles or view angles, during one revolution of the x-ray source and detector. Alternatively, an array of x-ray source and detector can be arranged to completely surround the patient, thereby permitting the CT system to acquire a complete set of data or projections without rotational movement.
The resolution of a CT imaging system along its z-axis is an important performance parameter. The ability to resolve fine structures enables improved diagnosis. For example, improved resolution aids not only in IAC and extremity studies, but also for cardiac applications to inspect the integrity of stent structures.
CT imaging systems typically provide image resolution along the patient long axis (z-axis) within limits imposed by such factors as collimator aperture size, x-ray focal spot size, detector cell size, and geometry of the CT system. A minimum slice thickness for at least one known CT system is 1.25 millimeters, as determined primarily by detector element pitch size. In order to improve image resolution, it is desirable to reduce slice thickness to less than 1 millimeter, and to achieve such reduction with minimal impact on imaging system hardware. For CT systems with native slice thickness less than 1 mm (e.g., at least one known CT system provides 0.625 mm), it is desirable to reduce the slice thickness even further (e.g., to less than 0.5 mm).
To reduce slice thickness of a single-slice or dual-slice imaging system, portions of the detector element are irradiated and the image data, such as projection data or image data, is deconvolved, to reduce the full-width-at-half maximum (FWHM) interval of the reconstructed slice profile. Difficulties can arise, however, in implementing this approach for a multi-slice imaging system collecting more than two detector row signals simultaneously because it is very difficult to design a pre-patient collimator to partially block the x-ray beam for each individual detector row.
Past efforts at improving spatial resolution in the z-axis for multi-slice imaging systems have focused primarily on hardware solutions, such as dicing the detector cells smaller or dynamically deflecting the x-ray focal spot to achieve improved sampling. Another approach that has been proposed for improving spatial resolution in the z-axis is the “thin twin” approach, in which a multi-slice detector is combined with a narrowly collimated x-ray beam to achieve thinner slice profiles than the aperture of the detector. Although these hardware-based approaches may improve resolution, these approaches increase the overall system costs, the complexity of the technology, and the acquisition time of the scanner.
Software-based solutions have also been proposed for improving z-axis spatial resolution. For example, various attempts have been made to use de-convolution techniques to reduce the slice sensitivity profile. Although these techniques may be effective in reducing the FWHM of the slice sensitivity profile, the techniques generally cause overshoot and undershoot in the processed images as a result of the characteristics of the de-convolution algorithms. The overshoot and undershoot phenomenon is highly undesirable, as it produces faulty structures around high-density objects and can potentially lead to clinical misinterpretation of the images.
Therefore, it would be desirable to design a system and method for improving spatial resolution in the z-axis of a multi-slice CT imaging system that overcomes the aforementioned drawbacks.